Smoke detectors of various types such as ionization and photo-electric are finding increasing use in both residential and industrial structures. These detectors are responsive to a predetermined concentration of smoke to provide an output signal to sound an alarm. For example, one type of detector, utilizing light reflected from smoke particles, onto a photo-responsive device, has circuit parameters such that an alarm is sounded when the smoke concentration reaches a density that will obscure between 1% and 2% of the light in a column of smoke 1 foot long.
Smoldering fires often take considerable time to produce a smoke concentration of 1%, and it would be desirable to have the alarm actuated at a lesser concentration, for example 0.2% to 0.4%. However, a smoke detector with such a low sensitivity is subject to false alarms from various causes, such as tobacco smoke, cooking smoke, dust, and industrial fumes.